1. Field of the Invention
The field of the present invention is internal mirror/internal resonator gas laser tubes.
2. Description of the Prior Art
At present, all internal mirror/internal resonator configurations of gas lasers employ forced or free air convection for dissipation of waste heat from the laser tube. Internal mirror/internal resonator gas lasers employ an envelope forming a gas discharge tube as a resonator support structure where laser mirrors are sealably mounted directly to the ends of the structure and maintained in alignment by the laser tube structure. The internal mirror/internal resonator arrangement has several advantages including simplicity in construction, ruggedness and dependability in operation and ease of maintenance. Historically, however, they have suffered from problems with resonator alignment as a result of thermal distortion of the plasma tube envelope due to operation at elevated temperatures relative to ambient conditions. This is especially true for higher power and/or longer versions of each of these laser types.
Continuous discharge gas lasers in which the gas is from the group consisting of argon, krypton, a mixture of argon and krypton, neon, and helium-cadmium, and helium-neon, are relatively energy inefficient in operation, resulting in the generation of significant waste heat which must be dissipated from the body of the laser tube. Argon, neon, krypton, a mixture of argon and krypton lasers operating in a continuous mode may generate waste heat on the order of fifty to two hundred watts per centimeter of bore length along the positive column or main bore of the laser discharge tube. Helium-cadmium lasers generate waste heat on the order of 4 watts per centimeter and helium-neon lasers about 0.4 watts per centimeter.
In general there are three types of laser tube and resonator configurations for these types of high heat generating gas and metal vapor discharge lasers: internal mirror/internal resonator; internal mirror/external resonator; and external mirror/external resonator types. In internal mirror/internal resonator laser tubes the laser mirrors are sealed onto the ends of the laser tube and therefore are internal to the laser tube. The resonator structure which maintains the laser mirrors in alignment with respect to each other and with respect to the gas discharge capillary of the laser tube is the laser tube structure itself. Therefore the term used to describe this type of resonator construction is internal resonator. In internal mirror/external resonator laser tubes the mirrors are mounted directly to the ends of the laser tube structure and form a hermetic seal. The alignment of these mirrors is however supported and maintained in alignment by an external resonator structure usually made of low thermal expansion materials such as Invar or quartz. Because of the usual difference in thermal expansion coefficient between the laser tube structural materials and the resonator materials the mirrors are normally mechanically isolated from the laser tube using a hermetically sealed bellows which is located between the mirror seals and the main portion of the laser tube. This bellows takes up the differential expansion between the laser tube and the external resonator which occurs due to heating of the laser tube. In external mirror/external resonator laser tube configurations the laser tube is sealed at the ends using windows which are disposed at the Brewster angle. The laser mirrors are external to the laser tube and not part of the hermetic envelope of the laser tube. The laser mirrors are supported in the external resonator in order to maintain alignment.
The choice of laser tube/resonator configuration depends upon the type of laser, its heat dissipation and the performance requirements including output power and optical stability characteristics. Each of the basic types of lasers described above has been available in all three types of tube/resonator configurations depending on the size of the laser. Small lasers for low output power applications normally use the internal mirror/internal resonator type of construction. This type of construction is typical of helium-neon, helium-cadmium, and low power argon-ion lasers. Higher power lasers and/or lasers with better beam pointing or transverse mode stability requirements utilize external resonators whether with internal or external mirrors. External resonators are needed to thermally and mechanically decouple the laser mirrors from the waste heat generated within the laser tube. Because an internal mirror/internal resonator laser tube is operating at elevated temperature compared to ambient, any slight variation in coolant distribution or temperature will cause lateral or axial temperature gradients along the surface of the laser tube. The most problematical temperature gradient in most internal mirror/internal resonator lasers is the lateral temperature gradient, dT, since it directly causes laser mirror rotational misalignments. Steady state temperature gradients which are independent of ambient temperature, input power level, or laser tube operating orientation can normally be compensated for by initial alignment of a laser. These gradients are considered controlled. Random temperature gradients are those which vary with ambient temperature, input power level, laser tube operating orientation or other random parameters. In general this random portion of the temperature gradient is proportional to the laser tube wall temperature. As the tube wall temperature decreases, so does dT. In the limiting case where there is no differential temperature between the tube envelope and ambient, there can be no steady state or transient lateral temperature gradients generated as a result of the cooling. Since thermal misalignment of laser mirrors is due to the magnitude of dT and independent of tube wall temperature, the lowest thermal misalignment will occur at the lowest tube wall temperature above ambient. The random gradients cause constant changes in the differential elongation of the one side of the tube with respect to the other side thereby causing angular rotation of the laser mirrors with respect to each other and with respect to the centerline of the gain capillary. This effect produces wander of the laser beam axis and, in severe cases, variations in the laser output power and mode shape. When a laser tube is subjected to a random lateral temperature gradient, dT, averaged across the length of the laser tube, one side of the laser tube grows a random amount which causes an approximately equal and opposite angular misalignment of the each laser mirror by an amount O.sub.D =aLdT/2D where a is the thermal expansion coefficient of the tube envelope material, L is the spacing between laser mirrors, dT is the random lateral temperature gradient, and D is the laser tube diameter. More elaborate formulations can be generated to describe complex structures where the random gradient varies along the length of the tube and/or the thermal expansion coefficient and/or diameter varies along the length of the tube as a result of the use of several different construction materials and geometry.
U.S. Pat. No. 4,625,317, issued on Nov. 25, 1986 to William P. Kolb and Dale E. Crane, entitled Internal Mirror Laser, teaches a thermal structure and cooling system for an argon-ion laser that is virtually free of thermal asymmetries which includes a laser cathode having a housing constructed of a material of high thermal conductivity and relatively low thermal expansion in conjunction with a cooling structure configuration which readily and uniformly dissipates heat. The support structure is fabricated of copper material of at least a minimal thickness and the cooling fins are disposed in one and two stages of radial fins at specified regions along the support structure forming the discharge tube and enclosed by a tubular thermally conductive band. The use of laser tube construction materials which have a high figure of merit equal to the ratio of thermal conductivity divided by thermal expansion coefficient for the cathode end bell as well as end caps of the laser tube. The high thermal conductivity of the cathode end bell allows circumferential conduction within the end bell and alleviation of random lateral temperature gradients within the cathode end bell.
None of the prior art patents or publications teaches the use of high thermal conductivity, high specific heat capacity liquids in contact with the external surface of the laser tube for the purpose of reducing random lateral and longitudinal temperature gradients in order to stabilize the resonator of an internal mirror/internal resonator laser tube thereby increasing the beam pointing, output power and transverse mode stability and the output power capability of internal mirror/internal resonator laser tubes.
U.S. Pat. No. 5,048,032, entitled Air Cooled RF Induction Excited Ion Laser, issued to John Ekstrand, John P. Goldsborough and David L. Wright on Sep. 10, 1991, teaches an ion laser which includes a resonant cavity and a heat dissipating mechanism. The resonant cavity includes an output coupler and a laser bore and contains an active medium. The heat dissipating mechanism dissipates heat which is associated with the laser bore during excitation of the active medium.
U.S. Pat. No. 5,048,043, entitled Gas Laser, issued to Wolfgang Welsch, Hans Krueger, Klemens Huebner and Rudolf Haeusler on Sep. 10, 1991, teaches a gas laser tube structure. U.S. Pat. No. 5,052,014, entitled Gas Laser Tube with Mask, issued to Masaaki Hiroshima and Yoshio Nakazawa on Sep. 24, 1991, also teaches a gas laser tube structure. U.S. Pat. No. 5,050,184, entitled Method and Apparatus for Stabilizing Laser Mirror Alignment, issued to George A. Nelson on Sep. 17, 1991, teaches an apparatus for stabilizing laser mirror alignment.
U.S. Pat. No. 4,953,176, entitled Angular Optical Cavity Alignment Adjustment Utilizing Variable Distribution Cooling, issued to John P. Ekstrand on Aug. 28, 1990, teaches varying the distribution of heat flow out of the plasma tube or structural element to compensate for any misalignment in response to position of the laser beam which controls misalignment of a laser beam due to bending of a plasma tube for an internal resonator ion laser, or another structural element supporting beam guiding optics. A detector is mounted along the optical path of the laser beam, and generates a position signal indicating drift of the laser beam from a preferred position. A cooling system, thermally connected to the structural material of the plasma tube or the structural element and connected to the detector, conducts heat out of the structural material in a controlled distribution in response to the position signal, so that misalignment of the optics due to thermal bending is minimized.
U.S. Pat. No. 4,987,574 issued on Jan. 22, 1991 to William R. Rowley and Patrick Gill, entitled Helium-Neon Lasers, teaches a stabilized helium-neon laser emits radiation in the ranges of ultra-violet, infra-red and visible other than red in at least two modes. A stabilizing system consists of an alignment heater which bends the laser tubes toward optimum alignment, a coil heater which cyclically varies the tube length and permanent magnets which reduce instability of mode polarizations and to optimize relative intensities of the modes. The transmitted output is stabilized in frequency by control of the laser tube length, with the stabilization signal derived from the steady or varying intensity of intensities of one or both of two orthogonally polarized optical outputs.
Another problem that occurs with lasers operating at high temperatures is that the tube bends and becomes misaligned. U.S. Pat. No. 4,010,363, entitled Laser Alignment System, issued to Karl Gerhard Herngvist on Mar. 1, 1977, teaches a laser alignment system which includes a gas discharge tube for producing the laser beam and a plurality of heaters each of which extends longitudinally along the exterior of the gas discharge tube spaced around the longitudinal axis of the gas discharge tube and which selectively heat the gas discharge tube. The laser alignment system is a rather complicated system which requires the comparison of each of the along the gas discharge tube. Using control circuits selective portions of the gas discharge tube may be heated when the laser becomes misaligned. The system is complex and requires positional misalignment data collection and feedback signals.
Referring again to internal mirror/internal resonator laser tubes, as the length of a laser tube design is increased, the tolerance to angular misalignment of the laser mirrors decreases. However, with increasing tube length, the angular misalignment of the laser mirrors increases due to random lateral temperature gradients induced in the tube by localized variations in the coolant temperature distribution, flow velocity and its distribution, and other factors. At some laser tube length the random deformation becomes greater than the tolerance to deformation and the beam pointing, output power and transverse mode stability degrades to an unacceptable level. Depending on the type of laser and the heat generation required to provide optical output, these lasers have been more or less limited in length and, as a result, laser output power. In addition, the more stringent the requirement on beam pointing, output power or transverse mode stability, the more limited the laser tube length and output power.
The precise length where a tube/resonator design must transition from internal to external resonator depends on many design factors such as the thermal conductivity, thermal expansion coefficient, geometry of the laser tube envelope, and laser mirror configuration, as well as performance factors such as beam pointing, output power and transverse mode stability. Three design parameters can be used to describe the design requirements for a stable internal mirror/internal resonator laser tube: the tolerance angle, .theta..sub.T ; the random thermal deformation angle, .theta..sub.D and the critical tube length, L.sub.C, which is defined as that length where the tolerance angle, .theta..sub.T is equal to the random thermal deformation angle, .theta..sub.D.
U.S. Pat. No. 4,233,568, entitled Laser Tube Mirror Assembly, issued to Randolph W. Hamerdinger and Robert C. McQuilan on Nov. 11, 1980, teaches a laser tube assembly which includes a laser tube and pair of laser mirrors. The laser tube has a hard glass to metal sealed laser resonator which is internal to the plasma tube for use in a helium-cadmium laser. Each laser mirror is sealed to one end of the laser tube. The sealant is able to withstand the relatively high temperatures which are utilized to remove contaminants during fabrication thereof. The sealant is also able to minimize gas permeation therethrough during utilization of the laser tube. U.S. Pat. No. 3,904,986, entitled Gas Laser Tube, issued to Karl Gerhard Hernqvist on Sep. 9, 1975, teaches a gas laser tube which includes an elongated envelope, an active laser medium, an output coupling mirror assembly which is disposed on one end of the elongated envelope and a reflector mirror assembly which is disposed on the other end thereof. U.S. Pat. No. 4,149,779, entitled Internal Laser Mirror Alignment Fixture, issued to Randolph W. Hamerdinger on Apr. 17, 1979, teaches a laser tube mirror alignment fixture.
U.S. Pat. No. 4,224,579, entitled Metal Vapor Laser Discharge Tube, issued to Calvin J. Marlett, Edwin A. Reed, Richard C. Johnson and William F. Hug on Sep. 23, 1980, teaches a metal vapor laser which includes an envelope, a capillary tube, an anode, a cathode and a pair of mirrors. The metal vapor laser discharge tube also includes a reservoir of helium, an evaporator and a condenser. The evaporator is fluidly coupled to the capillary tube adjacent to the anode. An active material is placed in the evaporator. The condenser is fluidly coupled to the capillary tube adjacent to the cathode. The laser discharge tube further includes a heater. The heater is mechanically coupled to the evaporator. The heater applies heat to the active material in the evaporator in order to produce a vapor the positive ions of which the cathode draws to the condenser.
Prior art patents have suggested various techniques to improve heat dissipation efficiency in external mirror/external resonator laser. However, none of these prior art patents relate to the method disclosed here of using liquids to stabilize the performance of internal mirror/internal resonator gas discharge laser tubes.
U.S. Pat. No. 4,715,039, entitled Internal Resonator Water Cooled Ion Laser, issued to Mike F. Miller and Kim M. Gunther on Dec. 22, 1987, teaches a laser which includes a resonator tube, a magnet and a plasma tube. The magnet is concentrically mounted in a spaced relationship within the resonator tube defining between the magnet and the resonator an outer coolant channel. The plasma tube is concentrically mounted in a spaced relationship within the magnet defining between the magnet and the resonator an inner coolant channel. The mirrors are then mounted externally to the plasma tube and are supported by an external resonator structure.
U.S. Pat. No. 4,897,851 issued on Jan. 30, 1990 to David L. Vecht and Shinan-Chur S. Sheng, entitled Water Cooled Laser Housing and Assembly, teaches an external mirror/external resonator which includes a concentrically arranged plasma tube, magnet and resonator tube. The mirrors and associated optics are housed at opposing ends of the resonator tube and are mounted on rods extending from respective ends of the resonator tube. The use of laser tube geometry and tube construction materials stabilizes the beam pointing direction of an external mirror/external resonator argon-ion laser.
U.S. Pat. No. 3,763,442, entitled Ion Laser Plasma Tube Cooling Device and Method, issued to William H. McMahan on Oct. 2, 1973, teaches a device which cools an ion laser plasma tube which is based on the utilization of a thermal conductor which is adapted to become fused to an ion laser plasma tube in such a manner as to efficiently transfer the heat which is generated to a surrounding cooling medium, such as air, while mechanically adjusting to differential thermal expansion and contraction of the ion laser plasma tube.
U.S. Pat. No. 4,081,762, entitled Gas Laser with a Laser Capillary Positioned in a Discharge Tube, issued to Hans Golser and Helmut Kindl on Mar. 28, 1978, teaches an improvement to a gas laser the components of which have substantially equal coefficients of thermal expansion. The gas laser includes a vacuum-tight discharge tube, a laser capillary, an anode, a cathode, electrical discharge generator and two mirrors which form an optical resonator.
U.S. Pat. No. 4,953,172, entitled Gas Laser, issued to Thomas R. Gurski on Aug. 28, 1990, teaches a gas laser with a discharge bore which is defined by a single-bore extruded ceramic discharge tube is disclosed. An outer tube is located over the discharge tube so as to define an annular space therebetween. Caps are located over both ends of the tubes and each cap is provide with at least one gas transport passage so the discharge bore and the annular space are in communication. The annular space is filled with electrically insulating, thermally conducting components such as washers, baffles and ceramic granules. When the laser is in operation the annular space serves as a gas return path so a uniform equilibrium pressure is maintained in the discharge bore. The components in the annular space inhibit the flow of electrical current in the space so all of the current flow is through the discharge bore so as to excite the gas therein. The components in the annular space also provide a thermally conductive path between the discharge tube and the outer tube to diffuse heat away from the discharge bore.
U.S. Pat. No. 4,477,907, entitled Low Power Argon-ion Gas Laser, issued to William H. McMahan on Oct. 16, 1990, teaches a gas laser which achieves low output power by using a plasma guide and mirror configuration which restricts lasing action to a portion of the resonator cavity at relatively high conversion efficiency.
U.S. Pat. No. 4,481,633, entitled Wet-jacket Argon-ion Laser, issued to William H. McMahan on Nov. 6, 1984, teaches a segmented ceramic tube which forms an external mirror/external resonator argon-ion laser discharge tube. The segmented ceramic tube is fluid cooled and constructed to avoid shunting of current through the coolant.
In operation, a laser of the external mirror/external resonator, such as an argon laser, generates considerable heat, which must be removed, at least in applications where stability of laser output is critical. The heat can cause thermal expansion of the resonator structure and can adversely affect the mirror alignment and thus the laser output. Water cooling of the laser is frequently utilized to minimize this problem.